Meaning of the Words Compass and Protractor

Date: 08/17/99 at 22:45:01
From: Heather Bugbee
Subject: Compasses and protractors

Dear Dr. Math,

Why is a compass called a compass when it protracts and retracts, and 
why is a protractor called a protractor when it is used to measure 
degrees? I know this is not really a high school math question, but it 
intrigues me a great deal.

Thank you for your time.
Heather Bugbee

Date: 08/18/99 at 13:13:31
From: Doctor Peterson
Subject: Re: Compasses and protractors

Hi, Heather. This is not exactly a math question, but I like to talk 
about words too!

The word "compass" means "to go around, encircle"; it originally meant 
"to measure" by pacing (passus). Since a compass "goes around," this 
makes some sense.

"Protract" originally meant "to drag out, lengthen." How is that 
related to what a protractor does? My dictionary gives one definition 
as "[surveying] To draw to scale by means of a scale and protractor." 
I suspect that because scale drawings or maps make heavy use of angles 
(remember the AAA similarity theorem: a scale drawing is similar to 
the original), the protractor was originally thought of as a tool for 
protracting in this sense.

Words don't have to make sense to be used, but I think we can see at 
least a little sense in the history behind these words.

- Doctor Peterson, The Math Forum
  http://mathforum.org/dr.math/